For many polyolefin applications, including films and fibers, flexibility and softness combined with retention of properties at high end-use temperatures are desirable attributes. In other polyolefin applications, including those that involve injection molding and rotomolding fabrication techniques, toughness is a critical attribute, particularly low temperature toughness and impact resistance. A low melt viscosity (high melt flow rate) is advantageous for almost all polyolefin fabrication processes, because this reduces cycle time or allows for lower temperature and/or energy requirements.
For polyethylene-type resins, the most common approach to improving flexibility and toughness is to lower the crystallinity (and therefore the density) by addition of comonomer. However, this typically also results in reduced melting points. Traditional approaches to achieve low melt viscosity are lowering the molecular weight and broadening the molecular weight distribution of the resin. However, both approaches can have detrimental effects on the final physical properties of the polyolefin article, such as lower puncture resistance or lower impact resistance. What is needed is a method to improve physical properties, such as flexibility and toughness, while simultaneously lowering melt viscosity. It would also be further advantageous in a fabrication environment be able to continuously vary these parameters to match changing needs, instead of choosing between discrete polyethylene types sold by density, melt index, and composition.
Addition of a plasticizer or other amorphous substance to a polyolefin is one way to attempt to address these needs. Some patent disclosures directed to such an end are U.S. Pat. Nos. 4,960,820; 4,132,698; 3,201,364; WO 02/31044; WO 01/18109 A1; and EP 0 300 689 A2. These disclosures are directed to polyolefins and elastomers blended with materials such as mineral oils which contain aromatic and/or other functional groups. Typically, addition of mineral oil also lowers the melt viscosity because the mineral oil itself has a viscosity well below that of the polyolefin.
Addition of compounds like mineral oils tend to improve the flexibility of a polyolefin, which identifies such compounds as “plasticizers” under the commonly accepted definition; that is, a substance that improves the flexibility, workability, or distensibility of a plastic or elastomer. Mineral oils are also often used as extenders, as well as for other purposes, in polyolefins. However, use of these additive compounds typically does not preserve the optical properties (e.g., color and or transparency) of the polyolefin, among other things. The melting point of the polyolefin is also typically not preserved, which reduces the softening point and upper use temperature of the composition. In addition, such additive compounds often have high pour points (greater than −20° C., or even greater than −10° C.), which results in little or no improvement in low temperature toughness of the polyolefin.
To improve the low temperature characteristics, it is customary to choose lower molecular weight, amorphous compounds as plasticizers. Low molecular weight compounds are also chosen for their low viscosity, which typically translates into lower melt viscosity and improved processibility of the polyolefin composition. Unfortunately, this choice often leads to other problems. For example, all or some of the additive can migrate to a surface and evaporate at an unacceptably high rate, which results in deterioration of properties over time. If the flash point is sufficiently low (e.g., less than 200° C.), the compound can cause smoking and be lost to the atmosphere during melt processing. It can also leach out of the polyolefin and impair food, clothing, and other articles that are in contact with the final article made from the plasticized polyolefin. It can also cause problems with tackiness or other surface properties of the final article. What is needed is a compound which imparts superior low temperature properties while also exhibiting low migration, leaching, and/or evaporation behaviors.
Another shortcoming of typical additive compounds is that they often contain a high (greater than 5 wt %) degree of fuinctionality due to carbon unsaturation and/or heteroatoms, which tends to make them reactive, thermally unstable, and/or incompatible with polyolefins, among other things. Mineral oils, in particular, consist of thousands of different compounds, many of which are undesirable for use in polyolefins due to molecular weight or chemical composition. Under moderate to high temperatures these compounds can volatilize and oxidize, even with the addition of oxidation inhibitors. They can also lead to problems during melt processing and fabrication steps, including degradation of molecular weight, cross-linking, or discoloration.
These attributes of typical additive compounds like mineral oils limit the performance of the final plasticized polyolefin, and therefore its usefulness in many applications. As a result, they are not highly desirable for use as modifiers for polyolefins. What is needed is a modifier that does not suffer from these deficiencies. Further, the modifier should improve the flexibility and toughness of the polyolefin, while maintaining its melting point. Ideally, the modifier has a low pour point, while still of sufficient molecular weight to avoid unacceptable exudation and extraction. It should also not contribute to deterioration of optical properties, surface properties, thermal stability, and or oxidative stability, and the like.
It would be particularly desirable to modify polyolefins such as polyethylene by using a simple, non-functionalized compound such as a paraffin. However, it has been disclosed that aliphatic or paraffinic compounds would impair the properties of polyolefins, and was thus not recommended. (See, e.g., CHEMICAL ADDITIVES FOR PLASTICS INDUSTRY 107-116 (Radian Corp., Noyes Data Corporation, NJ 1987); WO 01/18109 A1).
Other examples of polyolefins combined with plasticizers include: WO 2004/014998 which discloses blends of propylene based polymers with various non-functionalized plasticizers; WO 98/44041 which discloses plastic based sheet like material for a structure, especially a floor covering, which contains in a blend a plastic matrix comprising a chlorine free polyolefin or mixture of polyolefins and a plasticizer characterized in that the plasticizer is an oligomeric polyalphaolefin type substance; and U.S. Pat. No. 4,536,537 which discloses blends of LLDPE (UC 7047), polypropylene (7522) and Synfluid 2CS, 4CS, or 6CS having a viscosity of 40 to 6.5 cSt at 100° F./38° C., however the Synfluid 4CS and 6CS are reported to “not work” (col 3, ln 12).
Other background references of interest include EP 0 448 259 A, EP 1 028 145 A, U.S. Pat. Nos. 4,073,782, 3,415,925, 5,869,555, 4,210,570, 4,110,185, GB 1,329,915, U.S. Pat. Nos. 3,201,364, 4,774,277, JP 01282280, FR 2094870, JP 69029554, Rubber Technology Handbook, Werner Hoffman, Hanser Publishers, New York, 1989, pg 294-305, and Additives for Plastics, J. Stepek, H. Daoust, Springer Verlag, N.Y., 1983, pg-6-69.
Certain mineral oils have been classified as Hydrocarbon Basestock Group I, II, or III by the American Petroleum Institute (API) according to the amount of saturates and sulfur they contain and their viscosity indices. Group I basestocks are solvent-refined mineral oils that contain the highest levels of unsaturates and sulfur, and low viscosity indices; they tend to define the bottom tier of lubricant performance. They are the least expensive to produce and currently account for the bulk of the “conventional” basestocks. Groups II and III basestocks are more highly refined (e.g., by hydroprocessing) than Group I basestocks, and often perform better in lubricant applications. Group II and III basestocks contain less unsaturates and sulfur than the Group I basestocks, while Group III basestocks have higher viscosity indices than the Group II basestocks do. Additional API basestock classifications, namely Groups IV and V, are also used in the basestock industry. Rudnick and Shubkin in Synthetic Lubricants and High-Performance Functional Fluids, Second edition (Marcel Dekker, Inc. New York, 1999) describe the five basestock Groups as typically being:    Group I—mineral oils refined using solvent extraction of aromatics, solvent dewaxing, hydrofining to reduce sulfur content to produce mineral oils with sulfur levels greater than 0.03 weight %, saturates levels of 60 to 80 weight % and a Viscosity Index (VI) of about 90;    Group II—mildly hydrocracked mineral oils with conventional solvent extraction of aromatics, solvent dewaxing, and more severe hydrofining to reduce sulfur levels to less than or equal to 0.03 weight % as well as removing double bonds from some of the olefinic and aromatic compounds, saturate levels are greater than 95-98 weight% and VI is about 80-120;    Group III—severely hydrotreated mineral oils with saturates levels of some oils virtually 100%, sulfur contents are less than or equal to 0.03 weight % (preferably between 0.001 and 0.01 weight %) and VI is in excess of 120;    Group IV—“polyalphaolefins,” which are hydrocarbon liquids manufactured by the catalytic oligomerization of linear alpha-olefins having 6 or more carbon atoms; in practice, however, this Group is generally thought of as synthetic basestock fluids produced by oligomerizing alpha-olefins have 4 or more carbons; and    Group V—esters, polyethers, polyalkylene glycols, and includes all other basestocks not included in Groups I, II, III, and IV.
Prior attempts of adding mineral oils to polyethylenes to modify properties involve for the most part addition of Group I and Group II mineral oils. Even in cases where the mineral oil is not identified by an API Group classification, such as the case for so-called “process oils,” “technical white oils,” “food grade oils,” etc., such mineral oils are still readily categorized into two classes based on Viscosity Index alone: those with VI less than 120 (similar to Group I and Group II mineral oils), and those with VI of 120 or greater. Certain aspects of the present invention ideally pertain to substances with a VI of 120 or greater, which excludes Group I and Group II mineral oils and any other mineral oils with VI<120.
We have discovered that certain hydrocarbon modifiers (preferably certain liquids), preferably comprising branched paraffins, will advantageously plasticize polyethylene to improve physical properties of polyethylene and reduce its melt viscosity, without compromising melting point and resin molecular weight, and without suffering from the deficiencies typically obtained with mineral oils. Moreover, addition of these liquid hydrocarbon modifiers provides a means to change such properties on a continuous scale, based on real-time needs, which is typically not possible due to the availability of only discrete polyethylene grades. Furthermore, a different set of relationships between physical and thermal attributes is obtained, compared to those available from traditional polyethylenes of different densities and composition, which allows for new and advantageous properties of the fabricated articles.